The weakness is a part of the long run recovery process, but given that you say you are fairly new to running in general you may be running too much for your conditioning right now. It sounds like you don't realistically have the leg strength specific to running to handle the hours of pounding on the long runs. This doesn't mean you should take a whole week off, but you do need to alter whatever plan you are following to allow muscle cell repair.

My advice is to back off running, but not completely. Supplement your aerobic training with less stressful exercises like water jogging, swimming, biking. Cut down to two midweek runs that should be on the shorter side. Try not to skip long runs. As you peak with longer and longer long runs, there should be easier weeks in between with full days off of running. I won't kid you, long runs stink and they do hurt, but if the hurt continues for more than 3 days you should probably adjust your schedule.

Honestly, I think you can absolutely do this if you got through the 18 miles. Finishing a marathon will give you a great sense of accomplishment, but if it were easy the reward would not be so great.